Redundant Members. 469
Also E B = C D = 1474.
and weight couple = righting couple
Tons. Feet. Tons. Feet.
5X44'22 = 1474X15
22FI = 22I'I
Case (i). Stress in tie rod from dtoa becomes - 15 4-5 = 10 tons —
Case (2). Stress in jib from e to a becomes H-i6-|4-5 = 2i|tons +
other stresses being unaltered. The advantage of Case (i) is obvious.
If the barrel be between d and e the stress must be resolved on the
two members. As the load varies from nothing to a maximum, the
righting moment of balance weight should be half the maximum.
Redundant Members are such as receive no stress accord-
ing to force diagram, but contribute usually to resist buckling.
In Fig. 429, cross-members connect weak strut B with strong tie A,
but otherwise receive no stress. In Fig. 430 A is redundant, but
receives stress due to instability of strut. B. (SeeApp. IIL,p. 923.)
Example 38.—A crane is constructed as in Fig. 431. Draw the
stress diagram for internal and external forces. (Hons. Mach. Constr.
The crane is shewn at a, and the stress diagram found at
mencing with the weight E D.